riscv: lib: Fix ZBB strnlen reading past count boundary

The ZBB-optimized strnlen loop loads one word ahead before checking the
aligned boundary:

    REG_L   t1, SZREG(t0)       // load next word
    addi    t0, t0, SZREG       // advance
    orc.b   t1, t1
    bgeu    t0, t4, 4f          // boundary check AFTER load

where t4 = (s + count) & -SZREG.  When s is aligned and count is a
multiple of SZREG, t4 equals s + count and the loop loads a full word
starting at exactly s + count.  If s + count falls on a page boundary
with the next page unmapped, this faults.

Fix by computing the aligned boundary from the last valid byte
(s + count - 1) instead of s + count.  This makes the loop stop at the
word containing the last valid byte rather than potentially loading the
word after it.  The count == 0 case is already handled by the beqz
early exit.

Also add a pre-loop guard (bgeu t0, t4) for the case where all valid
bytes fit within the first word.  With the adjusted boundary, t4 can
equal t0, and entering the loop with stale register state from the
first-word processing would produce incorrect results.

The final minu clamp ensures the result is still correct when the last
loaded word extends past s + count - 1 within the same aligned word.

Fixes: 5ba15d419fab ("riscv: lib: add strnlen() implementation")
Signed-off-by: Michael Neuling <mikey@neuling.org>
Assisted-by: Claude Opus4.6 High Thinking
Link: https://patch.msgid.link/20260413010738.1622423-1-mikey@neuling.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>

diff --git a/arch/riscv/lib/strnlen.S b/arch/riscv/lib/strnlen.S
index 53afa7b5b314d9a16e5fdce6d24d876e0dec0b01..a8911605c24801b64b5b6e648084290dae14600a 100644 (file)
--- a/arch/riscv/lib/strnlen.S
+++ b/arch/riscv/lib/strnlen.S
@@ -83,8 +83,13 @@ strnlen_zbb:
        sub     t3, t3, t2
        slli    t2, t2, 3
 
-       /* Aligned boundary. */
+       /*
+        * Aligned boundary.  Use the address of the last valid byte
+        * (s + count - 1) to avoid loading a word past the count
+        * boundary in the loop below.  count == 0 is handled above.
+        */
        add     t4, a0, a1
+       addi    t4, t4, -1
        andi    t4, t4, -SZREG
 
        /* Get the first word.  */
@@ -120,6 +125,9 @@ strnlen_zbb:
 
        bgtu    t3, a0, 2f
 
+       /* All remaining bytes are in the first word, no loop needed. */
+       bgeu    t0, t4, 2f
+
        /* Prepare for the word comparison loop. */
        addi    t2, t0, SZREG
        li      t3, -1